Which MPG for comparrison

[wipe_out]

Managed to get out on the new boat for a good number of hours this weekend and was looking at some economy numbers while working through the break in period that are generated by the instruments but when I started thinking about comparisons it occurred to me that "MPG" is a bit of a minefield because there are potentially 4 combinations of "MPG".. With "Nautical Miles", "Statute Miles, "Imperial Gallons" and "US Gallons"..

I assume that most would base their numbers on Nautical miles right?

What about the "Gallons"? Do most use imperial or US?

In the end I decided the nautical mile / litre number seemed the least open to interpretation but that didn't help much either because most numbers quoted are "MPG"..

 

[Momac]

Litres per hour is commonly quoted figure.
I generally figure out how long a journey will take and then work out how many litres I should need .
Or perhaps given a number of hours run work out how much I should need to put in the tank .

 

[Deleted User YDKXO]

NMPG - nautical miles per imperial gallon is the most common unit in the UK for fuel consumption. Some people like to think in terms of galls per hour GPH or litres per hour LPH but IMHO they're not very useful unless related to distance, hence NMPG

 

[Bandit]

you want litres burnt going through the water per nautical mile. its the only sensible measure.

I don't rate fuel burn rates on engine electronics.

When you next fill up how many litres have you used and how many miles have you done through the water since your last fill?

End of converstation.

Ignore fuel burn rates and miles over the ground, its irrelevant.

 

[gjgm]

Managed to get out on the new boat for a good number of hours this weekend and was looking at some economy numbers while working through the break in period that are generated by the instruments but when I started thinking about comparisons it occurred to me that "MPG" is a bit of a minefield because there are potentially 4 combinations of "MPG".. With "Nautical Miles", "Statute Miles, "Imperial Gallons" and "US Gallons"..

I assume that most would base their numbers on Nautical miles right?

What about the "Gallons"? Do most use imperial or US?

In the end I decided the nautical mile / litre number seemed the least open to interpretation but that didn't help much either because most numbers quoted are "MPG"..

Not that I do so other than fag packet fuel need calcs, but litres per (nautical) mile. Mind you, most gauges have a mind of their own, so it gets a bit of a mystery sometimes.
So, easy cruising in calm weather I might get 2.25LPM, and 2.5+ harder driving/weather.
In the end, the boat uses what it uses and I try to keep the gauge out of the red, and thats about it !
Edit. Of course none of this bears any resemblance to the official figures and it isnt even a VW engine

 

[wipe_out]

I think I will give up on the comparison idea.. Seems the only comparison I can make is to my own boat over time using which ever numbers I choose as the base..

 

[jfm]

imho, nmpg (UK gallons) and litres per mile make sense. Personally I think in litres per mile and my instruments show that figure.

Litres per hour is of course only half a story and if you want to manage your economy it makes sense to think in litres per mile. For example, in a P boat, when you speed up while already on the plane your litres per hour might rise dramatically but so might your speed, almost in proportion, so that your litres per mile doesn't change significantly

IMHO, if you calculate using only the data when you refill with fuel, you get an average of idling, harbour speeds and open water speeds, and you wont know the mix (ususally) so you cannot sensibly compare your data. You need instantaneous value for lpm, in different operating conditions, in order to predict range or decide whether you can go from A to B without needing to stop for fuel

 

[benjenbav]

imho, the information is useful in two contexts:

How much fuel do I need to get to where I am going/can next refuel?

Am I burning significantly more fuel than I used to for the same sort of thing?

For both it is useful to think of the units which make sense to you.

For example, if you think the distance to Plymouth is 175 NM rather than 200 statute miles or 325 km it's easiest to use NM.

Likewise, if you think of fuel in terms of how many litres you buy, that's the easiest unit.

In other words, no right answer: go for the one that makes life easiest.

Tonnes per parsec, anyone?

 

[gjgm]

IMHO, if you calculate using only the data when you refill with fuel, you get an average of idling, harbour speeds and open water speeds, and you wont know the mix (ususally) so you cannot sensibly compare your data.

I suspect alot of day boating is pretty similar, especially around the Solent as 1. there is not very far to go and 2. the harbour,river,lock aspect is a given for each trip. Mind you I do remember being amazed a while back that some forumite was getting stunningly efficent figures for his heafty flybridge til he fessed up he rarely went beyond a trip from Chi Marina and into the harbour